1. Field of the Invention
This invention relates to circuit arrangements for controlling electric motors.
2. Introduction to the Invention
Electric motors are used -in numerous applications to rotate, translate or otherwise control the position or orientation of the objects being so controlled. For example, in automobiles, electric motors are used to control the position or orientation, or both, of windows, mirrors, mirror assemblies, seats, convertible tops, antennas and headlights, to name just a few.
Similar to the distinction between analog and digital in the context of electrical signals, computers, and the like, such applications of electric motors may be viewed as having analog and digital functions. For example, in some applications, e.g. power seats and mirrors, the object being positioned can be moved to rest at any desired position within two extremes. Such applications may be viewed as analog applications. Seats may be moved as desired between a full forward and a full rearward position, between a full downward position and a full upward position and/or between a full tilt forward position and a full tilt backward position. Similarly, power mirrors may be rotated between limits about one or more axes.
In other applications, e.g. headlights and mirror assemblies, the object being positioned can be moved to rest at one of two extremes. Such applications may be viewed as digital, or, more accurately, binary applications. Binary, because they rest in one of two possible states. Headlights may be rotated to a fully visible position when the lights are to be turned on, or the headlights may be rotated to a fully hidden position when the lights are to the turned off. A mirror assembly may be rotated to a full inward position, e.g. to facilitate parking in close quarters, or the mirror assembly may be rotated to a full outward position when the automobile is to be driven.
In this context of analog and binary position control, power windows are a hybrid. Power windows may be moved up or down, stopping at any desired position between a full upward position and a full downward position. In addition, the "one touch down" feature available with modern power windows permits the driver to cause the window to move to a full downward position with just a momentary flick of a switch. Hence, a hybrid system combines features of both analog and binary systems. Although power window applications are typically configured to apply the one touch feature in only one direction (and therefore may be thought of as a combination of analog and binary features), other hybrid applications may combine analog control with one touch control in two directions. Therefore, as used herein, hybrid systems include applications which combine analog control with one touch control in one or two directions.
Motorized position control systems which operate in a binary or hybrid mode are configured to automatically stop at the extreme positions. In order to be able to automatically stop, such motorized position control systems need a means to detect that the object being positioned, e.g. window, mirror assembly, antenna, etc., has reached the extreme position, and then to interrupt current to the actuating motor.
As used herein, interrupting current is meant to include applications in which current is totally disconnected in order to stop a motor, and applications in which current is substantially reduced, but not totally disconnected, in order to stop a motor.
Systems implemented to control motors in binary and hybrid applications tend to be complex and expensive, and, because of their complexity, can become unreliable. With respect to one touch down power windows, one known approach employs a small resistive element to monitor current flow with an operational amplifier circuit or custom semiconductor device arranged to detect the voltage across the resistive element. Upon detection of the voltage exceeding a threshold corresponding to the motor stall current, the circuit interrupts current to the motor. Such circuits may not be temperature compensated, and may possibly result in either nuisance stopping, i.e. stopping before the window is completely open, or in failing to stop the motor. To protect from the latter case, and prevent burning the motor, such systems may include a back-up timing circuit to interrupt current to the motor in a predetermined time if the current has not otherwise already been interrupted.
As used herein, the terms "binary application" and "binary mode" are interpreted to have a common meaning and are interchangeable as may be appropriate in the context used. Likewise, the terms "hybrid application" and "hybrid mode" are also interpreted to have a common meaning and are interchangeable as may be appropriate in the context used.
U.S. Pat. No. 4,678,975 (Vrabel et al.) discloses a control system for a reversible D.C. window drive motor that allows for one touch operations of the motor to lower the associated window.
Copending, commonly assigned U.S. patent application Ser. No. 08/564,465, now U.S. Pat. No. 5,864,458, discloses useful electrical protection systems which can be produced by connecting a PTC element in series with a mechanical switch or other circuit interruption element, and by connecting a bypass element in parallel with the PTC element and the circuit interruption element. When an overcurrent passes through such a system, the PTC element increases in resistance, and as a result an increased current passes through the bypass element. The bypass element is functionally linked to the circuit interruption element so that the increased current through the bypass element converts the circuit interruption element into its fault state.
PTC circuit protection devices are well known. The device is placed in series with a load, and under normal operating conditions is in a low temperature, low resistance state. However, if the current through the PTC device increases excessively, and/or the ambient temperature around the PTC device increases excessively, and/or the normal operating current is maintained for more than the normal operating time, then the PTC device will be "tripped," i.e. converted to a high temperature, high resistance state such that the current is reduced substantially. Generally, the PTC device will remain in the tripped state, even if the current and/or temperature return to their normal levels, until the PTC device has been disconnected from the power source and allowed to cool. Particularly useful PTC devices contain a PTC element which is composed of a PTC conductive polymer, i.e. a composition which comprises (1) an organic polymer, and (2) dispersed, or otherwise distributed, in the polymer, a particulate conductive filler, preferably carbon black. PTC conductive polymers and devices containing them are described, for example in U.S. Pat. Nos. 4,237,441, 4,238,812, 4,315,237, 4,317,027, 4,426,633, 4,545,926, 4,689,475, 4,724,417, 4,774,024, 4,780,598, 4,800,253, 4,845,838, 4,857,880, 4,859,836, 4,907,340, 4,924,074, 4,935,156, 4,967,176, 5,049,850, 5,089,801 and 5,378,407, the disclosures of which are incorporated herein by reference for all purposes.
While PTC devices are commonly placed in series with a load to act as a "resettable fuse," the non linear characteristics of a PTC device make it useful in circuit arrangements in which the PTC device may be used, for example, as a current sensor, and act in coordination with other circuit devices to control the state of the circuit.